I had the same problem with my 2001 Navigator. It is the bladder system - probably one of them has a leak. Nav's have a bladder system which is used to raise and lower the car. Because of the cost in replacing the bladder system (sorry can't remember now what they quoted me but it was high), I decided to replace the bladder system with plain old shocks. Cost me less than $100...course I got free labor in this case but just the cost of the bladders was 4 times this cost.